Centrosome duplication entails the formation of a single procentriole next to each centriole once per cell cycle. The mechanisms governing procentriole formation are poorly understood and constitute a fundamental open question in cell biology. We will launch an innovative multidisciplinary research program to gain significant insight into these mechanisms using C. elegans and human cells. This research program is also expected to have a significant impact by contributing important novel assays to the field. Six specific aims will be pursued: 1) SAS-6 as a ZYG-1 substrate: mechanisms of procentriole formation in C. elegans. We will test in vivo the consequence of SAS-6 phosphorylation by ZYG-1. 2) Biochemical and structural analysis of SAS-6-containing macromolecular complexes (SAMACs). We will isolate and characterize SAMACs from C. elegans embryos and human cells, and analyze their structure using single-particle electron microscopy. 3) Novel cell-free assay for procentriole formation in human cells. We will develop such an assay and use it to test whether SAMACs can direct procentriole formation and whether candidate proteins are needed at centrioles or in the cytoplasm. 4) Mapping interactions between centriolar proteins in live human cells. We will use chemical methods developed by our collaborators to probe interactions between HsSAS-6 and centriolar proteins in a time- and space-resolved manner. 5) Functional genomic and chemical genetic screens in human cells. We will conduct high-throughput fluorescence-based screens in human cells to identify novel genes required for procentriole formation and small molecule inhibitors of this process. 6) Mechanisms underlying differential centriolar maintenance in the germline. In C. elegans, we will characterize how the sas-1 locus is required for centriole maintenance during spermatogenesis, as well as analyze centriole elimination during oogenesis and identify components needed for this process